To learn more about the antecedents and immediate precursors of sudden cardiovascular death, we have examined 338 men and 16 women at high risk plus 109 matched controls and have followed them prospectively for five years at 6 month intervals with reexaminations and tape recordings of the electrocardiograms. 50 deaths have occurred of which 35 have been arrhythmic. Subjects experiencing sudden arrhythmic death have been characterized by evidence of myocardial ischemia, hypertrophy, dilatation, or congestive heart failure at initial examination, plus evidence of ventricular dysrhythmias, delays of abnormal pattern of QRS conduction or disorders of sinus node with sustained slow heart rates. Most of these subjects also had major "risk factors" including hypertension, hyperlipidemia, diabetes mellitus, present cigarette smoking, severe airway disease, or high alcohol intake. Both myocardial disease and disorders of rhythm and conduction advanced during the observation period to death. Increased myocardial disease has been associated with prolongation of QT interval and shortening of coupling interval of VPCs. Findings suggest that early cycle VPCs occurring in "vulnerable period" of repolarization initiate fatal dysrhythmia in many cases. In one case recorded at time of death this phenomenon has been documented.